FALLOUT 3: main story about 30 hours; story with some extras about 52.5 hours; 100% completion about 175 hours. These are IGDB player-submitted averages from 6 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for FALLOUT 3 yet. Tracked platforms include PlayStation.
Game Tales keeps what happened between you and a game in one Tale you can keep building.